This invention relates to the determination of engine mass air-flow and more particularly to a method and means for determining the mass of air available for combustion within a cylinder of a crankcase scavenged two-stroke engine as a function of the pressure, volume, and temperature of the air undergoing a substantially polytropic phase of compression in a crankcase chamber, prior to its transfer to the cylinder combustion chamber.
In a crankcase scavenged two-stroke engine, each cylinder has a separate crankcase chamber into which air is inducted during a portion of the engine operating cycle. This inducted air is compressed during part of the engine cycle, when the crankcase chamber is decreasing in volume due to the downstroke of the piston within the cylinder, and is then transferred to the cylinder combustion chamber where it is mixed with fuel for ignition.
In order to effectively control the emission and performance characteristics of crankcase scavenged two-stroke engines, it is necessary to know the mass of air available at the time of combustion within a cylinder. Once such information is known, critical engine parameters, such as spark advance, fueling requirements, and injector timing, can be adjusted to achieve the desired emission and performance objectives.
Mass air-flow sensors are commercially available, and have been used with internal combustion engines in the past to provide the required information regarding the air mass available for combustion. However, at the present time, mass-air flow sensors with sufficient accuracy are relatively expensive as compared to other sensors used for engine control.
One prior technique for determining cylinder mass air in a crankcase scavenged, two-stroke engine is described in U.S. application Ser. No. 377,383, filed July 10, 1989, which is copending with the present application and assigned to the same assignee. There, the mass of air available for combustion within a cylinder is obtained by determining the mass of air undergoing compression within a crankcase chamber, prior to its transfer to the associated cylinder combustion chamber. In determining the air mass in the crankcase chamber, the pressure of the air undergoing compression in the crankcase must be integrated with respect to the decreasing crankcase volume. Consequently, a numerical integration of sampled crankcase pressure is required when implementing the technique via a computerized engine control system. Although this prior technique satisfactorily eliminates any need for a mass air-flow sensor in the control of the two-stroke engine, it does require the integration of several samples of crankcase pressure, employing a significant amount of computer processing time, which is generally at a premium with the present state of the engine control art. The present invention provides an alternative mass air-flow measuring technique.